Fuses with built-in indicating plungers



2,913,551 FUSES WITH BUILT-IN INDICATING PLUNGERS Frederick J. 'Kozacka, South Hampton, N.H., assignor to The Chase-Shawmut'Company, Newburyport, Mass.

Application November 21, 1958, Serial No. 775,524 6 Claims. (Cl. 200-121) .Miniature or subminiature power fuses call for very .short fusible means made of a metal such as preferably silver, or possibly copper, which has a relatively. small fusing energy and a relatively-high conductivity. 1

' Indicating plungers are I normally held in position against the bias of a spring by a fusible element in wire form, known as restraining wire, generally made of steel, in order to obtain the high degree of tensile strength required for the purpose in hand. The fusible element formed by the steel wire shunts the fusible-means of silver or copper which normally carries substantially the entire current of the circuit controlled by the fuse. The resistance of the steelwire is so high that it carries hardly any current as long as the fusible element of silver or copper is intact. After destruction by fusion and areing of the current path formed by the elementof-silvcr or copper, the voltage prevailing across the steel wire becomes sufficiently high to cause the flow of a-current of. suflicient..magnitude through the steel wire to cause rapid fusion thereof. The voltage gradient-along the are formed by fusion and back-burning of the steel wire tends to be relatively small, the vapors of steel being a relatively good conductor .of electricity'at the temperature around the boiling point'of steel. Because :the volt-- -age..gradient along an areresulting from vaporization of ,steel wire, even if relatively thin, tends to be' relatively low, the required minimum length for the fusible indicator plunger restraining wireof steel may exceed the maximum length permissible for the current-carrying fusible element or ribbon fuse link of silver or copper, i.e. the length'not to. be exceeded if the required currentcarrying capacity is to be maintained.

The prior art method to cope with substantially different requirements in regard to the length of the currentcarrying fusible element and in regard to the lengthof the fusible restraining wire for. a blown fuse indicating plunger. consists in housing the former and the latter in two separate casings of which one maybe mounted on the other such as shown, for instance, in Figs. 1 and 2 of United States Patent.2,794,,095 to Frederick J. Kozacka,gMay 28, 1957, Striker Pin Structures.. This'arrangement is, however, too bulky for many applications such'as, for instance, the protection of individual germanium .and .silicon rectifier cells by individual cell fuses calling for. a high degree of .compactness. It is, therefore, one object of this invention to provide. fuseshaving a plunger type blown fuse indicator ice which fuses are sufficiently compact to be used for the protection of individual germanium and silicon rectifier cells and which fuses have a built-in rather than an additional external indicating plunger.

Another object of the invention is to provide small power fuses having built-in blown fuse indicator plungers wherein the abutment means precluding the plungers from coming off the fuse structure upon fusion of the restraining wires for the plungers are arranged inside extremely compact fuse casings in such a way as not to be impaired by arcing.

Another object of the invention is to provide small current-limiting power fuses having indicator plungers and restraining wires therefor inserted into the circuit Without resorting to solder joints.

Still another object of the invention is to provide fuses wherein the conventional connector means in form of knife blade contacts are substituted by another kind of connector means which lend themselves particularly well to" accommodating indicator plungers and to accommodating restraining wires for such plungers though the length of these wiresgreatly exceeds the lengths of the fusible current-carrying elements or fuse links of silver or copper.

Still another object of the invention is to provide fuses with built-in indicator plungers including simple and effective means for precluding :the pulverulent arc-quenching filler in such fuses from interfering with the free movement of the indicator plungers or obstructing the free movement of the latter.

Other objects and advantages of the invention will, in part, be obvious and in part appear hereinafter.

For a more complete understanding of the invention reference may be had to the following detailed description thereof taken in connection with the accompanying drawing wherein Fig. 1 is a top plan view of a fuse embodying the present invention,

Fig. 2 is substantially a longitudinal section of the structure shown in Fig. l;

Fig. 3 is a bottom view of the structure shown in Figs. 1 and 2; and

Fig. 4 shows on a larger scale the same elements as shown on the left side of Fig. 2. g

All figures refer to a current-limiting subminiature power fuse intended primarily forthe protection of highly critical power diodes such as silicon diodes and germaniumdiodes, and-all figures are drawn on an enlarged scale. The particular structure shown has a current rating of 250 amps.'at 130 volts.

Referring now to the drawing, numeral 1 has been applied to indicate a pair of metallic terminal elements which are press-fitted into a tubular casing 4 of insulating material. Terminal elements or plugs 1 are held in position by angularly displaced steel pins 4a, shownin Figs. 2 and 3, omitted in Fig. 1. Each plug 1 is pro vided with an outer tubular extension arranged in coaxial relation to casing 4 and projecting beyond casing 4 in a. direction longitudinally thereof. As shown in Fig. 2, the extension 1a on the left plug 1 is screw-threaded and accommodates the indicating plunger 7 and a num ber of parts associated with said plunger, whereas the extension lb'on the right plug 1 forms a passage adapted for insertion ofa resilient metal cap 5b recessed in the right plug 1. Casing 4 houses a pair of fuse links 2 in form of silver ribbons. Silver ribbons 2 interconnect conductively the axially inner surfaces of plugs 1. T0

axially outer ends of links 2. Indicator plunger 7 is provided on the axially outer end thereof with a rounded head and forms a perforated flat tab on the axially inner end thereof. Cap 10 is mounted on plunger 7 and rests against a shoulder formed by the rounded head of the latter. The helical plunger-biasing spring 11 surrounding plunger 7 is inserted into cap 10. Cap 10 is, in turn, slidably inserted into a cap 9 which is press-fitted into tubular extension la. Cap 9 forms an axially inner support for spring 11 and has an opening therein forming a slide bearing for the passage of the axially inner end of plunger 7. Cap 8 which is likewise pressfitted into tubular extension In accommodates the end of plunger 7 passing through the opening'provided for that purpose in cap 9. Cap 8 has a perforationadjacent the axially inner end thereof through which perforation a plunger-restraining wire 6, preferably made of steel, is threaded. As clearly shown in Fig. 2 the plunger-re straining wire 6 forms a substantially V-shaped loop. The converging end of this loop is formed where the wire 6 is threaded through the perforation in cap 8 and through the axially inner flattened end of plunger 7. The ends of wire 6 remotefrom plunger 7 are inserted into a bore formed in the right plug 1 (Fig. 2) and clamped in position therein by means of resilient metal cap b. Reference numeral 3 has been applied to indicate a pulverulent arc-quenching filler, preferably quartz sand. This filler 3 fills the space inside of casing 4 and inside of extension 1a and is separated from plunger 7 by cap 8, which cap thus precludes interference of filler 3 with the free movement of plunger 7. It is apparent from Fig. 2 that the body of filler 3 extends beyond the left end of casing 4 and beyond the axially outer end surface of left plug 1 to a point of tubular extension 1a where indicator plunger 7 is located. The right terminal element or plug 1 (Fig. 2) is provided with an eccentric bore into which the resilient cap 5a is press-fitted. This latter bore is intended for filling the fuse with the pulverulent arc-quenching filler 3, and cap 5a forms a seal plugging the filling hole after filling of the fuse has been completed.

It will be apparent from the foregoing that the extension 1a, in addition to forming a connector and fuse support, lends itself well to accommodating a restraining wire whose length by far exceeds that of the fusible elements 2 intended to normally carry the current in the circuit which is controlled by the fuse. The force exerted by spring 11 may be relatively moderate-even if plunger 7 is intended to perform some mechanical work in addition to its indicating functionbecause filler 3 is precluded from frictionally engaging plunger 3 andfrom exerting a braking action on the latter once restraining wire 6 has melted and released plunger 7. The extension 1b might be omitted, if desired, and substituted by another connector means. It will be apparent that restraining wire 6 is connected into the circuit without resorting to any solder joint by the intermediary of caps 5b, 8 and 9. Parts 9 and 10 jointly form a housing for spring 11. v

As long as the current in the circuit protected by the fuse is not excessive, virtually all of the current is carried by fusible elements 2 which then effectively shunt restraining wire 6. The voltage across restraining wire 6 increases suddenly upon fusion of fusible elements 2 and build-up of a substantial arc voltage across the gap formed by fusion of elements 2 and subsequent arcing. As a result, a current of suificient magnitude then flows through restraining wire 6 to cause .rapid fusion of the latter. This, in turn, causes release of plunger 7 which is then projected from the right to the left (Figs. 2 and 4) under the action of biasing spring 11, thus indicating that the fuse has blown and capable of operating any device-cg. a tripping mechanism-supposed to operate in response to blowing of the fuse.

It will be understood that I have illustrated and de' scribed herein a preferred embodiment of my invention, and that various alterations may be made in the details thereof without departing from the spirit and scope of the appended claims.

I claim as my invention:

1. An electric fuse comprising a tubular casing of insulating material, a pair of metal plugs closing the ends of said casing, each of said pair of plugs having an axially inner end surface and an axially outer end surface, one of said pair of plugs having an axially outer tubular extension projecting beyond said casing in a direction longitudinally thereof, fusible element means inside said casing conductively interconnecting said axially inner end surfaces of each of said pair of plugs, a spring-biased indicator plunger arranged inside said tubular extension adjacent the axially outer end thereof, a plunger-restraining-wire attached to said plunger extending from one of said pair of plugs into the inside of said tubular extension on the other of said pair of plugs, and a pulverulent arc-quenching filler occupying the space between said axially inner end surface of each of said pair of plugs and extending into said tubular extension to a point axially outwardly from said axially outer end surface of one of said pair of plugs.

2. An electric fuse comprising a tubular casing of insulating material, a pair of metallic terminal elements closing the ends of said casing, each of said pair of terminal elements having an axially inner end surface and an axially outer end surface, each of said pair of terminal elements having an outer tubular extension projecting beyond said casing in a direction longitudinally thereof, fusible element means in ribbon form inside said casing conductively interconnecting said axially inner surface of each of said pair of terminal elements, a springbiased indicator plunger arranged inside said tubular extension of one of said pair of terminal elements adjacent the axially outer end thereof, a plunger-restraining-wire inside said tubular extension of said one of said pair of terminal elements, clamping means for parts of said plunger-restraining-wire remote from said indicator plunger inserted into the other of said pair of terminal elements through said tubular extension thereof, and a body of a pulverulent arc-quenching filler inside said casing and inside said tubular extension of said one of said pair of terminal elements, said body of filler extending substantially beyond the axially outer end of said casing and substantially up to the point of said tubular extension where said indicator plunger is located.

3. An electric fuse comprising a tubular casing of insulating material, a pair of metallic terminal elements closing the ends of said casing, each of said pair of terminal elements having an axially inner end surface and an axially outer end surface, each of said pair of terminal elements having an axially outer tubular extension arranged in coaxial relation to said casing and projecting beyond said casing in a direction longitudinally thereof, silver ribbon means inside said casing conductively interconnecting said axially inner end surface of each of said pair of terminal elements, a spring-biased indi cator plunger arranged inside said tubular extension of one of said pair of terminal elements adjacent the axially outer end thereof, a steel wire normally restraining said plunger arranged inside said tubular extension of said one of said pair of terminal elements, a resilient metal cap recessed into the other of said pair of terminal elements and clamping a point of said steel wire re mote from said plunger against said other of said pair of terminal elements, and a body of quartz sand inside said casing and inside said tubular extension of said one of said pair of terminal elements, said body of quartz sand extending substantially beyond one axially outer end of said casing and substantially up to the point of said tubular extension where said indicator plunger is located.

4."An electric fuse comprising a tubular casing of in sulating material, a pair of metallic terminal elements closing the ends of said casing, one of said pair of terminal elements defining a substantially tubular passage, fusible element means inside said casing conductively interconnecting the axially inner surfaces of said pair of terminal elements, an indicator plunger mechanism arranged inside said passage, said mechanism comprising an indicator plunger, a helical plunger-biasing-spring, means forming a housing for said spring having an opening therein for the passage of the axially inner end of said plunger, a perforated cap accommodating the end of said plunger passing through said opening, and a plung er-restraining-wire attached to said plunger extending from the other of said pair of terminal elements into said passage in said one of said pair of terminal elements and through said perforated cap to said plunger, and said fuse further comprising a body of a pulverulent arcquenching filler inside said casing and inside said passage separated from said plunger by said cap.

5. An electric fuse comprising a tubular casing of insulating material, a pair of metallic terminal elements closing the ends of said casing, one of said pair of terminal elements having an outer tubular extension coaxial to said casing projecting beyond said casing in a direction longitudinally thereof, fusible element means in ribbon-form inside said casing conductively interconnecting the axially inner surfaces of said pair of terminal elements, an indicator plunger mechanism arranged inside said tubular extension adjacent the axially outer end thereof, said mechanism comprising an indicator plunger, a helical plunger-biasing-spring surrounding said plunger, a first cap press-fitted into said extension forming an axially inner support for said spring, said first cap having an opening therein for the passage of the axially inner end of said plunger, a second cap pressfitted into said extension accommodating the end of said plunger passing through said opening in said first cap, said second cap having a perforation adjacent the axially inner end thereof, and a plunger-restraining-wire attached to said plunger extending from one of said pair of terminal elements into the inside ofsaid tubular extension on the other of said pair of terminal elements and through said perforation in said second cap, and said fuse further comprising a body of a pulverulent arcquenching filler inside said casing and inside said tubular extension separated from said plunger by said second cap.

6. An electric fuse comprising a tubular casing of insulating material, a pair of metal plugs closing both ends of said casing, each of said pair of plugs having an axially outer tubular extension arranged in coaxial relation to said casing and projecting beyond said casing in a direction longitudinally thereof, silver ribbon means inside said casing conductively interconnecting the axially inner surfaces of said pair of plugs, an indicator plunger mechanism comprising an indicator plunger, a helical plunger-biasing spring surrounding said plunger, a first cap press-fitted into said extension of one of said pair of plugs, said first cap forming an axially inner support for said spring, and having an opening therein for the passage of the axially inner end of said plunger, a second cap press-fitted into said extension of said one of said pair of plugs accommodating the end of said plunger passing through said opening in said first cap, said second cap having a perforation adjacent the axially inner end thereof, a plunger-restraining-wire of steel attached to said plunger extending from one of said pair of plugs into the inside of said tubular extension on the other of said pair of plugs and threaded through said perforation in said second cap, and a resilient metal cap re cessed into the other of said pair of plugs clamping a point of said plunger-restraining wire remote from said plunger against said other of said pair of plugs, and said fuse further comprising a body of quartz sand inside said casing and inside said extension of said one of said pair of plugs separated from said plunger by said second cap.

References Cited in the file of this patent UNITED STATES PATENTS 2,142,991 Bennett Jan. 10, 1939 2,435,472 Schuck Feb. 3, 1948 2,794,095 Kozacka May 28, 1957 2,834,852 Swain et al. May 13, 1958 

